Serial plasma carcinoembryonic antigen (CEA) levels were measured in 167 patients with metastatic breast cancer treated with fluorouracil, doxorubicin hydrochloride, and cyclophosphamide (FAC). In 84 patients, the pretreatment CEA value was abnormal. Response rates and remission duration were similar in patients with normal and abnormal pretreatment levels. Carcinoembryonic antigen concentrations decreased in 94% of patients who responded to FAC therapy. The duration of response was 22 months for patients in whom the CEA levels normalized v nine months in those in whom it decreased but never returned to normal. Increasing CEA levels correlated with progressive disease in 87% of patients and preceded clinical evidence of progression in 77%. Serial CEA measurements can monitor response to chemotherapy, provide useful prognostic information of response duration, and detect progressive disease early.